A study of the phosphorus release potential of sewage sludge in entrained flow gasification

• Co-gasification of coal and sewage sludge in an entrained-flow gasifier analyzed. • Fuel mixtures, reactor temperatures, and operating pressures variated with CFD simulations. • A thermodynamics-based model predicts phosphorus release in gas and slag phases. • Reactor zones with high phosphorus release potential are identified for varying conditions. • Elevated temperature, low oxygen and reduced pressure enhance P release in gas phase. The treatment of sewage sludge is becoming an increasingly problematic issue when it is used as landfill, due to the presence of hazardous substances such as pharmaceutical residues and heavy metals. Furthermore, when sewage sludge is burned as biomass, it can result in air pollution and the loss of phosphorus. Nevertheless, sewage sludge gasification allows for thermal treatment with synthesis gas generation and phosphorus recovery in a single process step, resulting in the generation of a minimal quantity of carbon dioxide. In this study, a model for the release of phosphorus from sewage sludge was developed. The model is based on thermodynamic calculations from which a phosphorus release potential is derived as a function of temperature, oxygen partial pressure and global pressure. Moreover, the model is integrated in a CFD model for the analysis of reactive flow and the identification of reactor zones with phosphorus release potential. The present study focuses on a top-fired entrained flow gasifier with co-gasification of coal and sewage sludge. Based on these models, an extensive parameter study was conducted, taking into account the fuel mixture, the overall reactor temperature, and the operating pressure. This paper presents a detailed analysis of the conversion of feedstocks in the co-gasification of sewage sludge and the resulting potential for phosphorus recovery. The findings indicate that the release of phosphorus in the gas phase is enhanced by elevated temperatures, low oxygen concentrations and reduced operating pressures.